In general, some types of electronic devices are designed to use a current source to provide power or charging currents to one or more portions of the device. Additionally, such current sources can also be used to generate sensor or control signals for one or more portions of such devices. However, physical current sources generally fail to behave ideally and typically fail to provide a constant current at all times. Instead, the current that is provided typically varies over time, thus resulting in noise. In some devices, the magnitude of the noise may not affect the operation of the device. However, in other devices, the magnitude of the noise can be at sufficient levels to cause damage to the device or to cause the device to operate improperly. For example, in the case of a current source providing control or sensor signals, a sufficient amount of noise can result in the control system of the device inadvertently changing operational modes. In another example, the variation in current can result in overloading or overheating of a circuit, leading to reliability issues with such devices. In yet another example, the variation in current could result in improper charging of a battery or other charge storage device, leading to a reduction in the capacity or life of such devices.
As described above, one of the difficulties with the design of electronic devices is the non-ideal behavior of most current sources. That is, in most current source circuits, the voltages and/or currents therein may vary and can result in a time varying component, i.e., noise, appearing in the output current. In some cases, this noise can be significant depending on the configuration of the current source. For example, one common configuration for a current source is to utilize a voltage supply with a bipolar junction transistor (BJT) in a current source configuration using a resistor voltage divider network to provide a bias voltage for the base of the BJT from the voltage supply.
Unfortunately, such a configuration is susceptible to generation of significant output noise due to variations in the output of voltage supply and noise in the voltage supply lines. With respect to noise in a current source circuit, the BJT effectively operates as two types of amplifiers, each associated with one of the two current paths from the voltage supply to the load. In the first path, from emitter to collector, the BJT operates as a common base amplifier with a non-inverting gain. In the second path, from base to collector, the BJT operates as a common-emitter amplifier with an inverting gain. Typically, when a BJT current source is designed, the resistors in the voltage divider network and the resistance and load at the emitter and collector, respectively, are selected such that the gains in the two paths are approximately equal and opposite in polarity to cancel at least small amounts of noise. However, as greater amounts of noise are generated at the voltage supply, the gains become increasingly unequal, resulting in significant noise in the output current.